In modern mobile communication terminals, heterojunction bipolar transistors (HBT) are commonly used as a component of a radio-frequency amplifier module. HBTs are generally expected to meet the performance requirements of, for example, high efficiency, high gain, high breakdown voltage (high ruggedness upon load mismatch), and high output power. For use in second-generation cellular phones, HBTs with high ruggedness upon load mismatch are still in strong demand, but recently there has also been a need for HBTs with higher output power. For use in third- and fourth-generation cellular phones, not only HBTs with high power added efficiency but also ones with higher efficiency, high gain, and high output power are in demand. These trends indicate a growing need for higher outputpower HBTs in recent years.
Japanese Unexamined Patent Application Publication Nos. 2006-60221 and 2008-130586 disclose high outputpower HBTs, mentioning their structure. These HBTs have a substrate and a stack of subcollector, collector, base, and emitter layers on the substrate. The subcollector and collector layers serve as an n-type collector region, the base layer as a p-type base region, and the emitter layer as an n-type emitter region. The collector layer is a stack of multiple doped layers with graded donor-impurity concentrations, higher on the subcollector layer side and lower on the base layer side. The portion of the emitter region through which the emitter current actually flows is referred to as an intrinsic emitter region. In the base and collector regions, too, the current flows through the portions lying beneath the intrinsic emitter region. The structure formed by the intrinsic emitter region and the portions of the base and collector regions lying therebeneath is referred to as an intrinsic HBT.
The HBT illustrated in FIG. 1A of Japanese Unexamined Patent Application Publication No. 2006-60221 has a collector layer that includes first, second, and third n-type doped layers, from the closest to a subcollector layer. The first n-type doped layer has an impurity concentration of 7×1016 cm−3 or more and 10×1016 cm−3 or less and a thickness of 200 nm or more and 400 nm or less. The second n-type doped layer has an impurity concentration of 4×1016 cm−3 or more and 7×1016 cm−3 or less and a thickness of 200 nm or more and 400 nm or less. The third n-type doped layer has an impurity concentration of 0.5×1016 cm−3 or more and 4×1016 cm−3 or less and a thickness of 100 nm or more and 500 nm or less. The subcollector layer has an impurity concentration of 4×1018 cm−3 and a thickness of 400 nm.
In the HBT illustrated in FIG. 1C of Japanese Unexamined Patent Application Publication No. 2006-60221, the collector layer has first, second, third, and fourth n-type doped layers, from the closest to a subcollector layer. The first n-type doped layer has an impurity concentration of 7×1016 cm−3 or more and 10×1016 cm−3 or less and a thickness of 200 nm or more and 400 nm or less. The second n-type doped layer has an impurity concentration of 4×1016 cm−3 or more and 7×1016 cm−3 or less and a thickness of 200 nm or more and 400 nm or less. The third n-type doped layer has an impurity concentration of 0.5×1016 cm−3 or more and 4×1016 cm−3 or less and a thickness of 100 nm or more and 500 nm or less. The fourth n-type doped layer has an impurity concentration of 0.84×1016 cm−3 or more and 4×1016 cm−3 or less and a thickness of 100 nm or more and 500 nm or less. The subcollector layer has an impurity concentration of 4×1018 cm−3 and a thickness of 400 nm.
In the HBT illustrated in FIG. 20 of Japanese Unexamined Patent Application Publication No. 2008-130586, the collector layer has first, second, and third n-type doped layers, from the closest to a subcollector layer. The first n-type doped layer has an impurity concentration of 5×1016 cm−3 and a thickness of 200 nm. The second n-type doped layer has an impurity concentration of 1×1016 cm−3 and a thickness of 200 nm. The third n-type doped layer has an impurity concentration of 5×1015 cm−3 and a thickness of 600 nm. The subcollector layer presumably has an impurity concentration of 1×1018 cm−3, although this is speculation based on a description in an Example in the disclosure.
As can be seen, in these HBTs, the subcollector layer has a high impurity concentration, at least 1×1018 cm−3. In the fabrication of an HBT, it is a common practice to dope the subcollector layer to the highest technically possible impurity concentration at the moment to minimize the collector resistance Rc, between the ends of the collector electrodes and the center of the collector layer. The collector layer is usually doped to a relatively low concentration, 1/10 or less of that in the subcollector layer, for two purposes. One is to prevent the base-collector capacitance from being too great, which would affect the efficiency, gain, and other radio-frequency characteristics of the HBT, and the other to prevent the base-collector and collector-emitter breakdown voltages from being too low, which would cause the HBT to be broken when operated to full radio-frequency power, in which its output voltage amplitude peaks.
Increasing the output power of an HBT requires reducing both collector resistance and base-collector capacitance of the HBT. With the known technologies, however, it is difficult to increase the output power of an HBT by reducing both collector resistance and base-collector capacitance of the HBT. The following describes the reason with reference to FIGS. 11 and 12. FIGS. 11 and 12 illustrate known HBTs each having a subcollector layer 20, a collector layer 30, a base layer 40, an emitter layer 50, an intrinsic HBT 110, a capping layer 120, a contact layer 80, an emitter electrode 51, collector electrodes 21, and base electrodes 41.
As illustrated in FIGS. 11 and 12, the collector layer of the known HBTs has a multilayer structure, and this structure helps give the HBTs the desired base-collector, collector-emitter, and on-state breakdown voltages. The impurity concentration and thickness (typically, concentration distribution) of each doped layer constituting the collector layer 30 determine these breakdown voltages. The total thickness of the doped layers constituting the collector layer 30, however, has been disregarded, and some known HBTs have a collector layer 30 thicker than necessary for the desired breakdown voltages. The base-collector capacitance Cbc of a known HBT is composed of a depletion layer capacitance Cbcd, external capacitances Cbcex1, and external capacitances Cbcex2. The depletion layer capacitance Cbcd is formed between the base layer 40 and the collector layer 30, the external capacitances Cbcex1 between the base electrodes 41 and base layer 40 and the collector electrodes 21, and the external capacitances Cbcex2 between the base electrodes 41 and base layer 40 and the subcollector layer 20. The depletion layer capacitance Cbcd makes a relatively large contribution, but the other two types of external capacitances, Cbcex1 and Cbcex2, also make non-negligible contributions.
If the collector layer 30 has the smallest thickness necessary for the desired breakdown voltages, the base electrodes 41 and base layer 40 are close to the collector electrodes 21 and subcollector layer 20, as in FIG. 11. This means that the external capacitances Cbcex1, formed between the base electrodes 41 and base layer 40 and the collector electrodes 21, are large, and so are the external capacitances Cbcex2, formed between the base electrodes 41 and base layer 40 and the subcollector layer 20. As a result, the output power, and therefore the gain and efficiency, of the HBT are low.
Making the collector layer 30 thicker than necessary for the desired breakdown voltages is a way to avoid such large external capacitances Cbcex1 and Cbcex2. This, however, causes the problem of a large access resistance of the collector layer 30, a layer having a lower impurity concentration (about 1/10) than the subcollector layer 20. For better understanding of this problem, the following describes some major resistance components that contribute to the collector resistance Rc of a known HBT with reference to FIG. 12. The contact resistance between the collector electrodes 21 and the subcollector layer and the resistance of the collector electrodes are not illustrated. In the drawing, the resistance components contributing to the collector resistance Rc are expressed in a lumped element circuit for brevity, although it would be technically more accurate to use a distributed element circuit.
The collector resistance Rc is composed of external subcollector resistances Rscex, internal subcollector resistances Rscin, and access resistances Rscac and Rcac. The external subcollector resistances Rscex are the resistances the subcollector layer 20 has in the areas from the ends of the collector electrodes 21 to the ends of the collector layer 30. The inner subcollector resistances Rscin are the resistances the subcollector layer 20 has in the areas beneath the collector layer 30. The access resistance Rscac is the resistance to current flowing from the subcollector layer 20 to the active region of the intrinsic HBT 110. The access resistance Rcac is the resistance to current flowing from the subcollector layer 20 to the region of the smallest thickness necessary for the desired breakdown voltages. The sum of an external subcollector resistance Rscex and an inner subcollector resistance Rscin, referred to as an (Rscex+Rscin) resistance, has a width equal to the thickness of the subcollector layer 20 (typically between about 0.5 μm and about 1.5 μm) and a length equal to the horizontal distance between the end of a collector electrode 21 to the center of the intrinsic HBT 110 (typically between about 2 μm and about 4 μm).
The access resistance Rscac has the same width as the intrinsic HBT 110 (typically between about 2 μm and about 6 μm) and the same length as the subcollector layer 20 (typically between about 0.5 μm and about 1.5 μm). The contribution of the access resistance Rscac is therefore negligible compared with that of the (Rscex+Rscin) resistances. The contribution of the access resistance Rcac, however, cannot be ignored, because the access resistance Rcac, although identical in width to the intrinsic HBT 110 (typically between about 2 μm and about 6 μm), extends over a length of about 0.3 μm to about 0.7 μm in the collector layer 30, in which the impurity concentration is 1/10 or less of that in the subcollector layer 20. It should be understood that the length and width of a resistance as mentioned herein refer to the lengths of the resistance as measured parallel and perpendicular, respectively, to the direction of the flow of current.
The collector resistance Rc is therefore given by (Rscex+Rscin)/2+Rcac. This means that if the collector layer 30 is thicker than necessary for the desired breakdown voltages, the access resistance Rcac, the resistance to current flowing from the subcollector layer 20 to the active region of the intrinsic HBT 110, is accordingly large, and so is the overall collector resistance Rc. As a result, the on-state resistance Ron of the HBT cannot be lower than a certain limit, capping the output power of the HBT. Because of this tradeoff between the external capacitances Cbcex1 and Cbcex2 and the collector resistance Rc, it is difficult to increase the output power of an HBT by reducing both of them, as long as the known structure continues being used.